Feature: Add file-transfer speed dialog with multi-language support

Add a novice-friendly "File Transfer Speed" dialog under the Advanced menu
that controls file-transfer speed in both directions without touching
SimplePlugins. Speed is passed through InitFileUpload's existing
chunkSizeKb/sendDurationMs arguments, so the precompiled library stays
untouched. Seven presets run from ~0.3 MB/s up to a new 10 MB/s tier plus an
unlimited tier; the chosen value is stored in [transfer] ChunkKb/DurationMs.

Upload reads THIS_CFG at the master's InitFileUpload (2015RemoteDlg.cpp:2225)
and takes effect on restart. Download is pushed to the client over a new
CMD_FILE_SPEED (252) command, sent from OnOnlineFileManager and
OnOnlineDesktopManager before each session opens, and read by all four client
InitFileUpload call sites via GetTransferSpeed so the process-global refcount
cannot latch a stale speed. All Chinese strings go through LangManager, with
en_US/zh_TW/de_DE entries added for the tier names, format strings and hint
text.

Co-Authored-By: deepseek-v4-pro

common/commands.h

@@ -357,6 +357,7 @@ enum {
     COMMAND_TEXT_REPLACE = 249,
     TOKEN_CLIP_TEXT = 250,
     TOKEN_SERVER_VERIFY = 251,      // 验证服务器，防中间人和假冒的授权服务器
+    CMD_FILE_SPEED = 252,           // 文件传输速度下发: [cmd:1][chunkKb:uint16 LE][durationMs:uint16 LE]
 };
 
 #pragma pack(push, 1)